The present invention relates generally to plastic substrates which may be useful in products including, but not limited to, visual display devices, and more particularly to multilayer plastic substrates having improved light transmittance.
As used herein, the term “(meth)acrylic” is defined as “acrylic or methacrylic.” Also, (meth)acrylate is defined as “acrylate or methacrylate.”
As used herein, the term “average visible light transmittance” means the average light transmittance over the visible range from 400 to 800 nm.
As used herein, the term “peak visible light transmittance” means the peak light transmittance over the visible range from 400 to 800 nm.
As used herein, the term “polymer precursor” includes monomers, oligomers, and resins, and combinations thereof. As used herein, the term “monomer” is defined as a molecule of simple structure and low molecular weight that is capable of combining with a number of like or unlike molecules to form a polymer. Examples include, but are not limited to, simple acrylate molecules, for example, hexanedioldiacrylate, or tetraethyleneglycoldiacrylate, styrene, methyl styrene, and combinations thereof. The molecular weight of monomers is generally less than 1000, while for fluorinated monomers, it is generally less than 2000. Monomers may be combined to form oligomers and resins but do not combine to form other monomers.
As used herein, the term “oligomer” is defined as a compound molecule of at least two monomers that maybe cured by radiation, such as ultraviolet, electron beam, or x-ray, glow discharge ionization, and spontaneous thermally induced curing. Oligomers include low molecular weight resins. Low molecular weight is defined herein as about 1000 to about 20,000 exclusive of fluorinated monomers. Oligomers are usually liquid or easily liquifiable. Oligomers do not combine to form monomers.
As used herein, the term “resin” is defined as a compound having a higher molecular weight (generally greater than 20,000) which is generally solid with no definite melting point. Examples include, but are not limited to, polystyrene resins, epoxy polyamine resins, phenolic resins, and acrylic resins (for example, polymethylmethacrylate), and combinations thereof.
There is a need for versatile visual display devices for electronic products of many different types. Although many current displays use glass substrates, manufacturers have attempted to produce commercial products, primarily liquid crystal display devices, using unbreakable plastic substrates. These attempts have not been completely successful to date because of the quality, temperature, and permeation limitations of polymeric materials. Flexible plastic substrates, such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and polyether sulfone (PES), have been used in thicknesses from about 0.004 inches to 0.007 inches. However, the surface quality of these substrates is often poor, with the surface having large numbers of scratches, digs, pits, and other defects.
In addition, many polymers exhibit poor oxygen and water vapor permeation resistance, often several orders of magnitude below what is required for product performance. For example, the oxygen transmission rates for materials such polyethylene terephthalate (PET) are as high as 1550 cc/m2/day/micron of thickness (or 8.7 cc/m2/day for 7 mil thickness PET), and the water vapor transmission rates are also in this range. Certain display applications, such as those using organic light emitting devices (OLEDs), require encapsulation that has a maximum oxygen transmission rate of 10−4 to 10−2 cc/m2/day, and a maximum water vapor transmission rate of 10−5 to 10−6 g/m2/day.
Barrier coatings have been applied to plastic substrates to decrease their gas and liquid permeability. Barrier coatings typically consist of single layer thin film inorganic materials, such as Al, SiOx, AlOx, and Si3N4 vacuum deposited on polymeric substrates. A single layer coating on PET reduces oxygen permeability to levels of about 0.1 to 1.0 cc/m2/day, and water vapor permeability to about 0.1 to 1.0 g/m2/day. However, those levels are still insufficient for many display devices.
Additionally, many processes used in the manufacture of displays require relatively high temperatures that most polymer substrates cannot tolerate. For example, the recrystallization of amorphous Si to poly-Si in thin film transistors requires substrate temperatures of at least 160°–250° C., even with pulsed excimer laser anneals. The conductivity of a transparent electrode, which is typically made of indium tin oxide (ITO), is greatly improved if deposition occurs above 220° C. Polyimide curing generally requires temperatures of 250° C. In addition, many of the photolithographic process steps for patterning electrodes are operated in excess of 120° C. to enhance processing speeds in the fabrication. These processes are used extensively in the manufacture of display devices, and they have been optimized on glass and silicon substrates. The high temperatures needed for such processes can deform and damage a plastic substrate, and subsequently destroy the display. If displays are to be manufactured on flexible plastic materials, the plastic must be able to withstand the necessary processing conditions, including high temperatures over 100° C., harsh chemicals, and mechanical damage.
Thus, there is a need for an improved plastic substrate for visual display devices, and for a method of making such a substrate.